This invention relates to turbocharged internal combustion engines and more particularly to turbocharged internal combustion engines having exhaust gas recirculation (EGR) systems.
As is known in the art, high performance, high speed engines are often equipped with turbochargers to increase power density over a wider engine operating range, and EGR systems to reduce the production of NOx emissions.
More particularly, turbochargers use a portion of the exhaust gas energy to increase the mass of the air charge (i.e., boost) delivered to the engine combustion chambers. The larger mass of air can be burned with a larger quantity of fuel, thereby resulting in increased power, torque and fuel efficiency as compared to naturally aspirated engines.
As is also known in the art, EGR systems are used to reduce NOx emissions by increasing the dilution fraction in the intake manifold. EGR is typically accomplished with an EGR valve that connects the intake manifold and the exhaust manifold. In the cylinders, the recirculated burned exhaust gas acts as an inert gas, thus lowering the flame and in-cylinder gas temperature and, hence, decreasing the formation of NOx. On the other hand, the recirculated burned exhaust gas displaces fresh air and reduces the air-to-fuel ratio of the in-cylinder mixture.
A typical turbocharger includes a compressor and turbine coupled by a common shaft. The exhaust gas drives the turbine, which drives the compressor, which in turn, compresses ambient air and directs it into the intake manifold. Continuously variable geometry turbochargers (VGT) allow the intake airflow to be optimized continuously over a range of engine speeds. In diesel engines, this is accomplished by changing the angle of the inlet guide vanes on the turbine stator. An optimal position for the inlet guide vanes is determined from a combination of desired torque response, fuel economy, and emissions requirements.
As is known in the art, lean burn gasoline engines, for example direct injection (DI) stratified charge (DISC) gasoline engines, can potentially improve fuel economy and CO2 emissions over conventional stoichiometric port fuel injected (PFI) engines. DISC engines operate with stratified combustion at very high air-fuel ratios, leading to reduced pumping losses and improved fuel economy. High air requirements limit the stratified operating regime to relatively low engine loads. One recent paper has been published (xe2x80x9cTurbocharging the DI Gasoline Enginexe2x80x9d by M. Wirth et al., Society of Automotive Engineers World Congress, SAE-2000-01-0251, March 2000) suggesting a potential for improvement in fuel economy by using boost to extend the lean operating regime of DISC gasoline engines. While implementation of a turbocharger with continuously variable turbine vanes in a boosted stratified system may provide the best compromise in terms of fuel economy and delivery of recirculated exhaust gas (EGR), such continuously VGT turbochargers are relatively expensive and relatively non-robust when used with high exhaust temperature gasoline engines.
In accordance with the present invention, a method is provided for controlling an engine having a variable geometry turbocharger with at least two discrete positions and an exhaust gas recirculation (EGR). The method includes producing a control signal to the turbocharger. The signal has at least two discrete levels, corresponding in steady state to the two discrete positions of the turbocharger. One of such levels is selected to provide a proper flow of air through the turbocharger to an intake of the engine and such selected level is modulated between such two levels over time to provide, over time, a proper average pressure at an input of the EGR to enable a proper flow of exhaust gases from the exhaust of the engine through the EGR back to the intake of the engine.
With such method, while there is relatively instantaneous control of the exhaust pressure at the input of the EGR, and hence proper instantaneous control of EGR flow through the EGR, because of the relatively large inertia, and hence slower response time, of the turbocharger to the control signal the turbocharger maintains proper boost flow to the input of the engine with its accompanying improved fuel efficiency. Such method may be used for high temperature gasoline applications and diesel engine applications where less expensive VGTs may be desirable.
In one embodiment, a method is provided for controlling an engine having a turbocharger with at least two discrete positions and exhaust gas recirculation (EGR). The method includes producing a control signal for the turbocharger having two signal components, a first signal component having a level to drive the turbocharger to the either a desirable open or closed position selectively in accordance with engine operating conditions, and a second signal component comprising a train of pulses having a duty cycle selected to control time average pressure across the EGR.
In one embodiment, a method is provided for controlling an engine having a turbocharger with at least two discrete positions and exhaust gas recirculation (EGR). The method includes producing a control signal for the turbocharger having two signal components, a first signal component having a level to drive the turbocharger to the either a desirable open or closed position selectively in accordance with engine operating condition, and a second signal component modulating the first signal, such modulation being selected to control time average pressure across the EGR.
In one embodiment, a method is provided for controlling an engine having a turbocharger with at least two discrete positions and exhaust gas recirculation (EGR). The method includes producing a control signal for the turbocharger having two signal components, a first signal component having a level to drive the turbocharger to the either a desirable open or closed position selectively in accordance with engine operating conditions, and a second signal component modulating the first signal, such modulation being selected to control time average pressure across the EGR, such modulation having a bandwidth higher than the bandwidth of the turbocharger so that while the time average pressure at the EGR is changed substantially instantaneously, boost provided by the turbocharger is substantially invariant.
In accordance with one embodiment, a method is provided for controlling an engine having a turbocharger with at least two discrete positions and exhaust gas recirculation (EGR). The method includes producing a composite, bi-level control signal for the turbocharger. The composite signal includes a first signal component and a second signal component. The first signal component has either a first level or a second level. The first or second level is selected to provide either a relatively high or relatively low flow of air, respectively, through the turbocharger to an intake of the engine. The second signal component modulates the first signal component between the first and second levels to provide, over time, an average pressure at an input of the EGR to provide a proper flow of exhaust gases from the exhaust of the engine through the EGR back to the intake of the engine while the first signal component provides a proper flow of air through the turbocharger to the intake of the engine.
In accordance with one embodiment, a method is provided for controlling an engine having a turbocharger and exhaust gas recirculation (EGR). The turbocharger has a compressor portion and a turbine portion connected to the compressor portion. The inlet guide vanes of the turbine portion have at least two discrete positions. Exhaust from the engine is passed to an input of the EGR and an input to the turbine. Changes in a control signal to the turbine change pressure at the input of the EGR and changes flow through the compressor to the input of the engine. The method includes producing the control signal as a composite, bi-level control signal. The composite signal has a first signal component and a second signal component. The first signal component has either a first level or a second level. The first or second level is selected to provide either a relatively high or relatively low flow of air, respectively, through the turbocharger to an intake of the engine. The second signal component modulates the first signal component between the first and second levels to provide, over time, an average pressure at an input of the EGR to provide a proper flow of exhaust gases from the exhaust of the engine through the EGR back to the intake of the engine while the first signal component provides a proper flow of air through the turbocharger to the intake of the engine.
With such an arrangement, the variable geometry turbocharger (VGT) has only discrete positions, in one embodiment two discrete positions, and therefore such device is less costly and may be more robust to the high exhaust temperatures of gasoline engines. The discrete e.g., here two-position (i.e., a relatively open position and a relatively closed position) VGT operates with a continuously variable duty cycle that increases EGR delivery without sacrificing the fuel economy benefit of boosted operation.
More particularly, while the two-position VGT has only an open and a closed setting, it is able to effectively maintain proper EGR control by a variable duty cycle control signal. This type of device is useful for performance enhancement, particularly maximum power. Typically, the device is closed at low engine speeds to improve transient performance. At higher speeds, the device is opened to provide maximum flow through the turbine.
For areas of the DISC engine operating strategy where boost is used to extend the lean operating regime, the objective is not related to maximum power. Here boost is used to provide the airflow necessary to achieve lean air-fuel ratios (AFR) at higher speed and load conditions than is possible with a naturally aspirated engine, resulting in improved fuel economy.
In accordance with one feature of the invention, a variable duty cycle implementation is obtained with a two-discrete position VGT. With this implementation, the VGT is moved from open to closed position in a periodic fashion. Although EGR and fuel consumption may vary instantaneously, average EGR delivery can be modified while having little effect on average fuel consumption, because of the relatively non-responsiveness of the compressor and intake manifold to such periodic fashion.
The invention takes advantage of turbocharger lag to increase EGR delivery without sacrificing the benefit in fuel consumption achieved due to lean operation. More particularly, the response of the compressor portion of the turbocharger to the composite signal supplied to an inlet flow, for example inlet area control device for the turbine portion of the turbocharger is slower than the response to the exhaust pressure change at the input to the EGR. Thus, modulating the inlet flow control device for the turbine does not result in any significant change in the established condition of the compressor (i.e., boost to the intake of the engine) yet enables more rapid response in the pressure at the engine exhaust (i.e., at the input to of the EGR).
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.